The term "toners" as used herein designates the fine-grain, electrically chargeable powders used in electrophotography. Such toners are used for developing latent charge images, for example in photocopy machines. The toners comprise a mixture of natural and/or synthetic resins with a low melting point and resin-soluble or resin dispersible coloring materials. Also included are additives affecting the physical properties of the toner such as their direction of charge, their adhesion to recording material and their tendency toward agglomeration. Preferably the toner is a readily flowing powder that causes only small amounts of mechanical abrasion of the electrophotographic recording elements and is resistant to deterioration of its physical properties. The toner must further be able to transfer rapidly and completely from the recording material to an image-receiving material. The various desired toner properties are best obtained by a toner that has a spherical grain shape.
There are several known processes for producing toners. German patent disclosures Nos. 28 15 093 and 30 22 333 provide examples of processes for imparting spherical shape to toner particles which have already been reduced to the desired grain size. In one case the resin is melted and mixed with the other ingredients. The molten mass thus produced is sprayed to make the spherical grains. This procedure however is only useful with substances which become highly fluid on melting. A second known method is to dissolve the toner material in a solvent with a low boiling point and then spray the solution at a pressure of 10 to 15 bars. Heat is used to subsequently remove the solvent. In each case toner particles with a nearly ideal spherical shape are obtained. However, these processes are costly in terms of energy and difficult to operate.
In most cases however the toner material is mixed, cooled and then coarsely broken up. This material is further reduced to the desired grain size in a pulverizer such as a ball mill. The toner particles are thereafter subject to a heat treatment that bring the resin serving as a binder to its melting point. The surface tension of the melted particle causes the particle to take on a spherical shape. To achieve this result the toner particles are mixed with air to form an aerosol. The aerosol is directed in a cross-current or counter-current with a hot air stream as in German published patent application No. 19 37 651. Alternatively, the spherical shape may be obtained by using hot air to form a fluidized bed of toner particles, as in german patent disclosure No. 27 29 070.
A particular disadvantage of these hot air methods is that the air must be at a temperature of approximately 500.degree. C. At this temperature the toner particles readily stick together and form inseparable agglomerates. Further, melt incrustations build up on the walls of the apparatus and tubing and unwanted chemical transformations take place in the toner components.
It is an object of the present invention to provide a process for producing a spherical grain shape in fine-grain toners wherein the toners are in solid form (i.e. beneath the toner melting temperature) and without the use of solvents. This provides for a lower expenditure of energy than in known processes. The particles are to be treated in a fluidized material bed and the grain-size band of the toner particles is to be superfine grain size.